The present invention relates to rack and pinion steering gears and, more particularly, to hydraulic power-assisted rack and pinion steering gears.
A known rack and pinion steering gear includes a pinion gear that is rotatably mounted in a housing and is connectable with a steering wheel of a vehicle. A rack bar extends through the housing and has opposite end portions that are connectable with steerable vehicle wheels. The rack bar is moves longitudinally relative to the housing for turning the steerable wheels of the vehicle. Gear teeth formed on the rack bar are disposed in meshing engagement with gear teeth on the pinion gear. A hydraulic motor, when actuated, assists in moving the rack bar longitudinally relative to the housing. A yoke assembly is disposed in the housing to support and guide movement of the rack bar relative to the housing. The yoke assembly includes a yoke bearing having an arcuate surface across which the rack bar moves. A spring biases the yoke bearing against the rack bar.
When the hydraulic motor is actuated to move the rack bar to a desired position relative to the housing, hydraulic fluid exerts a force in a first direction on a piston that is attached to the rack bar. The force causes the rack bar to move in the first direction toward the desired position. An inertial force of the rack bar, when the rack bar is moving toward the desired position relative to the housing, tends to cause the rack bar to overshoot the desired position. When the rack bar overshoots the desired position, the hydraulic motor is actuated to apply a force in a second direction, opposite the first direction, to move the rack bar back toward the desired position. Again, the rack bar tends to overshoot the desired position and the hydraulic motor is again actuated to move the rack bar in the first direction. As a result, the rack bar continues to oscillate past the desired position. Oscillation of the rack bar past the desired position may produce a rattling noise or a small twitch in the steering wheel that may be felt by the driver. This is especially true when the vehicle contacts a discontinuity in the road surface, such as a pothole, that causes a reaction in the vehicle suspension that removes the resistance to rack movement. When the resistance to rack movement is removed, the velocity of rack movement increases and the magnitude of the overshoot increases.
A rack and pinion steering gear that dampens the movement of the rack bar to eliminate or reduce the overshoot that result from movement of the rack bar toward the desired position is desirable. By dampening movement of the rack bar, the rattling noise and the twitch may be eliminated.
The present invention is an apparatus for turning steerable wheels of a vehicle in response to rotation of a steering wheel. The apparatus comprises a housing. A rack bar is movable longitudinally relative to the housing for turning the steerable wheels. The rack bar has a portion that extends through the housing and opposite ends that are connectable to the steerable wheels. The portion that extends through the housing includes teeth. A pinion gear is located within the housing and includes teeth. Teeth of the pinion gear are in meshing engagement with teeth of the rack bar. The apparatus also comprises a hydraulic motor for, when actuated, moving the rack bar relative to the housing. A valve assembly is responsive to rotation of the steering wheel for directing fluid to the hydraulic motor. The valve assembly has an actuated position for actuating the hydraulic motor and an unactuated position for discontinuing operation of the hydraulic motor. The apparatus further comprises a mechanism for dampening longitudinal oscillations of the rack bar. The mechanism comprises a yoke bearing which contacts the rack bar with a variable pressure that is dependent upon a velocity of rack bar movement relative to the housing.
In a further aspect of the invention, the apparatus comprises a housing. A rack bar is movable longitudinally relative to the housing for turning the steerable wheels. The rack bar has a portion that extends through the housing and opposite ends that are connectable to the steerable wheels. The portion that extends through the housing includes teeth. A pinion gear is located within the housing and includes teeth. Teeth of the pinion gear are in meshing engagement with teeth of the rack bar. The apparatus also comprises a hydraulic motor for, when actuated, moving the rack bar from an initial position relative to the housing to a desired position relative to the housing in response to rotation of the steering wheel and a yoke assembly for supporting and guiding the rack bar relative to the housing. The yoke assembly includes a pressure chamber and a yoke bearing. The yoke bearing contacts the rack bar on a side of the rack bar opposite the pinion gear. Friction between the yoke bearing and the rack bar increases in response to an increase in fluid pressure in the pressure chamber. The apparatus further comprises a fluid source for supplying fluid to the pressure chamber of the yoke assembly. The fluid source increases fluid pressure in the pressure chamber in response to movement of the rack bar relative to the housing toward the desired position.
In yet a further aspect of the invention, the apparatus comprises a housing. A rack bar is movable longitudinally relative to the housing for turning the steerable wheels. The rack bar has a portion that extends through the housing and opposite ends that are connectable to the steerable wheels. The portion that extends through the housing includes teeth. A pinion gear is located within the housing and includes teeth. Teeth of the pinion gear are in meshing engagement with teeth of the rack bar. The apparatus also comprises a hydraulic motor for, when actuated, moving the rack bar relative to the housing. A valve assembly is responsive to rotation of the steering wheel for directing fluid to the hydraulic motor. The valve assembly has an actuated position for actuating the hydraulic motor and an unactuated position for discontinuing operation of the hydraulic motor. The apparatus further includes a yoke assembly for supporting and guiding the rack bar relative to the housing. The yoke assembly includes a pressure chamber and a yoke bearing. The pressure chamber receives fluid from the valve assembly. The yoke bearing contacts the rack bar on a side of the rack bar opposite the pinion gear. Friction between the yoke bearing and the rack bar increases as fluid pressure in the pressure chamber increases. Fluid pressure in the pressure chamber increases in response to movement of the valve assembly from the actuated position toward the unactuated position.